This is a very good point- consider all of the friction points that make self-checkout slow and cumbersome. How many of them apply to manned checkouts?
The weight thing is absolutely the most frustrating, and I would put money that it's not an effective theft deterrent.
I don't know if it's intentional, but the places around me seem to have largely solved the problem of cashiers being faster, by putting the slowest people on earth as cashiers...
What's weird is how many refuse to let you just enter the code on the sticker. You have to search through their stupid menu to find it, and it may not be what you actually have